Many nanomaterial molecules may act as a “host” to other “guest” molecules. At certain temperatures the guest molecule will come out of the host or rearrange itself to an ordered/disordered state. In order for the guest molecule to come out of the host it must absorb a large amount of energy. If the guest molecule goes from an ordered state to a disordered state inside the host molecule then it also must absorb large amounts of energy. Conversely, if the guest goes from a disordered to an ordered state it releases energy.
The encapsulation of guest molecules using zeolites, fullerenes, micelles and clathrates is well known. However, in these complexes, there is typically a relatively low amount of chemical binding to the host, and the host mass to guest mass ratio is extremely high. These factors limit the peak enthalpies of the complexes.
Organic and metal organic frameworks are self assembled via hydrogen bonding or metal coordination to create network topologies that act as a host to bind guest molecules that undergo reversible interactions with the host molecule. This class of compounds can be considered as a subset of supramolecular and coordination polymers. This subset that focuses on inclusion compounds has attracted much attention in recent years. These molecules are promising in regard to their fundamental and practical applications such as molecular recognition, crystal engineering, chemical sensing, new solid materials, drug delivery, chemical synthesis, gas storage, and separation science.
The design of nanomaterials with novel topology using molecular self assembly has been the focus of intense activity because these materials have high surface area and permanent porosity when the guest molecules are removed from the host network. Particularly, metal organic frameworks (MOFs) have been recently exploited for gas storage and separation applications. Like zeolites, metal-organic frameworks (MOFs) are crystalline hybrid materials consisting of open frameworks that can accommodate several different guest or refrigerant molecules. Different than zeolites, MOFs represent a new class of functional materials consisting of metal centers linked with organic building blocks to produce diverse and customizable structural frameworks.
These metal centers and organic linkers readily self-assemble into materials with open framework structures, where all the porosity is accessible for storage applications. Several porous materials, such as zeolites and activated carbons, have been reported for gas storage applications, but MOFs have received considerable attention over the past few years because of the high mass flux, thermal stability (in excess of 500° C. for some MOFs), adjustable chemical functionalities and pore sizes, extra high porosity, and availability of hundreds of well characterized materials reminiscent of zeolites.